Humans perceive light energy of varying wavelengths as color, detected by two types of light-sensitive receptors in the retina of the eye, rod cells and cone cells. The rod cells detect only the quantity of light, working at even low light levels with fewer photons, also known as night-vision. They are stimulated by the intensity of light and are responsible for perceiving the size, shape, and brightness of visual images, but do not perceive color and fine detail. The cone cells, of which there are three types, each capable of detecting a different range of wavelength of light being received, require hundreds of photons to activate them. They are less sensitive to low illumination levels but provide color information. Of the three types of cones, each one contains a distinctive type of pigment, absorbing red light, green light, and blue light.
By contrast, digital imagers comprise an array of pixel cells, each one of the pixel cells including a photoconversion device, e.g., a photodiode gate, photoconductor, or a photodiode, for converting light into an electrical charge. In a CMOS imager, a readout circuit is connected to each pixel cell, which typically includes a source follower output transistor. The photoconversion device converts photons to electrons which are typically transferred to a floating diffusion region connected to the gate of the source follower output transistor. A charge transfer device (e.g., transistor) can be included for transferring charge from the photoconversion device to the floating diffusion region. In addition, such imager cells typically have a transistor for resetting the floating diffusion region to a predetermined charge level prior to charge transference. The output of the source follower transistor is gated as an output signal by a row select transistor.
In color imagers, the pixel cells also have a color filter over the surface of the sensor, which limits the specific wavelengths of light that may be permitted to enter the photoconversion device. A Bayer pattern filter of alternatively red and green filters in a first row, and green and blue filters in a second row is most commonly used in a typical 2×2 square of pixel cells, which is repeated for the entire array, as illustrated in FIG. 1. There is an emphasis on green filters due to human visual response, which reaches a maximum sensitivity in the green wavelength region (550 nm) of the visible spectrum. Hence, when processed, the green data provides not only chrominance information, but as its peak response is close to the peak response of the human eye, it is also used for luminance information.
Color filtered pixels, like cone cells, require a greater number of photons, relative to unfiltered pixel cells or rod cells, in order to produce an appreciable signal. This is largely due to the color filters themselves, which diminish transmissivity. Color imagers must also have a near-infrared (NIR) blocking filter in the optical path to assure that NIR energy is not absorbed by the color filtered pixels. However, in low light conditions, not only does the imager suffer from the color filter transmissive losses, but it also cannot take advantage of the NIR energy present without an additional mechanism to remove the NIR filter from the pixel array.
A monochromatic imager is able to take advantage of the NIR energy. The pixel cells in a monochromatic imager have neither color nor NIR blocking filters, and are therefore more sensitive to lower levels of incident light, including NIR energy. However, monochromatic imagers are incapable of capturing color information of images.
There is a desire and need for a color imager that can provide better images in low light conditions.